Abstract Huanglongbing (HLB), previously known as citrus greening disease, is a devastating disease caused by gram negative, phloem-limited, fastidious bacteria Candidatus Liberibacter spp. HLB exists in nearly all commercially cultivated citrus, causing losses to growers by affecting tree vigor, production, fruit development, and quality. Recently, some promising HLB-tolerant germplasm has been identified, and showed particularly high concentration of metabolites, such as flavone, flavanone, aldehyde, and monoterpene. This review focuses on the citrus metabolic response against the HLB and we have summarized a comprehensive metabolic pathway that is activated in response to HLB. The antibacterial role of these metabolites, which were high in the HLB tolerant varieties are discussed. Generally, most of the amino acids, flavonoids, terpenes, and volatile compounds were significantly higher, even hundreds times of increase, in HLB-tolerant varieties. We also summarized the secondary metabolites, which were differentially altered in leaves, fruits, phloem sap (stem), and roots of infected citrus plants. Different metabolic studies have suggested that particular metabolites may play vital role in restricting the movement and multiplication of pathogens. Moreover, these metabolic signatures can be developed into tolerance markers against HLB. Genome-editing technologies should be used to confirm the functions of candidate genes responsible for increased production of compounds related to HLB tolerance. Engineering the metabolic pathways to create an ideal combination of gene alleles to propel metabolite flow for the antimicrobial activities, is an alternative tool to breed tolerant cultivars against HLB.

中文翻译：

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寄主物种对柑橘黄龙病（绿化病）的代谢机制

摘要 黄龙病（HLB），原名柑桔绿化病，是由革兰氏阴性、韧皮部限制性、苛养菌Candidatus Liberibacter spp引起的毁灭性病害。HLB 存在于几乎所有商业栽培的柑橘中，通过影响树木活力、产量、果实发育和质量给种植者造成损失。最近，一些有希望的 HLB 耐受种质被鉴定出来，并显示出特别高浓度的代谢物，如黄酮、黄烷酮、醛和单萜。本综述重点关注柑橘类针对 HLB 的代谢反应，我们总结了响应 HLB 激活的综合代谢途径。讨论了这些代谢物的抗菌作用，它们在 HLB 耐受品种中很高。一般来说，大部分的氨基酸、黄酮类、萜类、HLB 耐受品种的挥发性化合物含量明显更高，甚至增加了数百倍。我们还总结了次生代谢物，它们在受感染的柑橘植物的叶子、果实、韧皮部汁液（茎）和根中发生了差异性改变。不同的代谢研究表明，特定代谢物可能在限制病原体的移动和繁殖方面发挥重要作用。此外，这些代谢特征可以发展成针对 HLB 的耐受性标记。应使用基因组编辑技术来确认负责增加与 HLB 耐受性相关的化合物产量的候选基因的功能。设计代谢途径以创建基因等位基因的理想组合，以推动代谢物流动以实现抗菌活性，